The invention relates to an apparatus for opening and closing a bottom tap opening in a vacuum induction melting and casting furnace, with a movable closure body, a plug rod for example, as well as a drive unit for displacing the closure body.
There are many different tapping apparatus for melting and casting furnaces for controlling the flow of molten metals from a metallurgical vessel. For example, U.S. Pat. No. 5,083,689 describes an outlet system with a movable closure plug for controlling the flow of molten metals from a metallurgical vessel, having a control piston which is engaged in a bore and situated at the bottom end of a plug rod, and having a yoke connected with the upper end portion of the plug rod. A coupling red extending radially from the plug rod is present between the yoke and the plug rod, and for the connection between the yoke and the coupling rod at least one releasable clamping device permitting changes of angle and length is provided.
U.S. Pat. No. 4,736,930 discloses a closure system for a tap opening in the bottom of a metallurgical vessel, especially a metal melting furnace, with a shut-off means by which the tap opening can be closed from below, and which is protected against direct contact with the molten metal by a filler material placed in the tap opening. A tube is displaceable from a lowered blocking position to a raised release position releasing the tap opening, and has a protective layer of refractory material at least in the area of contact with the molten metal.
These known devices have the disadvantage that they do not allow a reliable opening and closing of the bottom pouring opening. This is especially necessary in the pouring of small amounts of high-purity metals. In the use of the above-mentioned plug rods consisting usually of metal in inductively heated melting and casting furnaces, an undesired heating of the plug rod occurs due to the excitation of the metal part of the rod by the induction field of the furnace.
The plug rods are usually actuated by hydraulic jacks which are disposed within the vacuum chamber. The oil fog caused by the piston rod of the hydraulic jack within the vacuum chamber is of no great importance in most cases. For the melting and casting of high-purity metal alloys, however, the oil fog within the vacuum chamber is a great disadvantage and can result in contamination of the molten metal.
The present invention provides reliable opening and closing of the bottom tap opening, while preventing undesired heating of the plug rod and contamination within the furnace by the accessory equipment, which may cause an oil fog.
According to the invention, the plug rod is made of ceramic material, and is connected by levers to a coupling means which has a compression spring to limit the maximum closing force of the closure body, and can be displaced by an electric motor drive. Advantageously, this permits a reliable opening and closing of the bottom tap opening, prevents the heating of the plug rod, and prevents interference with the melting of high-purity metal alloys by oil fog within the vacuum chamber.